What is Cancer?

What is Cancer?

cancer incidence graphCancer represents the abnormal growth of cells. It is a rare event if you think about how many cells there are in any one human body–about 30 trillion. And all it takes is one renegade cell to eventually form a tumor.

The body’s immune system is designed to be on the lookout for renegade cells, which are different in size and shape and divide faster than normal cells. However, some cancer cells can avoid detection from the immune system and grow into a tumor.

Learn more about cancer in the sections below.

What makes a renegade cell?

Each cell carries the entire set of genetic instructions for an organism. This is called the genome. These instructions are encoded in the genes contained in our DNA. If our genome is a book, the genes are sentences (such as “make my eyes brown”) and the DNA are the letters in the sentences. Cells go through a normal life cycle of division and death. This pattern can become disturbed by any number of things… chemicals, radiation, a virus… and damage can occur to the DNA of one of the newly divided cells. If the damage is severe, the cell will die. However, some damaged cells are still able to divide and multiply and may even mutate further, promoting its “wrongness.” These cells don’t behave like normal cells. They don’t organize like they’re supposed to and instead grow into bunches until a small benign tumor or group of cells (also called dysplasia) form.

It is when this group of cells breaks through the basement membrane (the thin sheet of fibers that underlies the epithelium lining cavities and surfaces of organs, including the skin, or the endothelium, which lines the interior surface of blood vessels), that this benign tumor becomes a malignant tumor—cancer.

cell structure drawing image

Cancer cells do not respond to regular cell growth, division, and death signals like they are supposed to. They grow unchecked and erratically. They do not go through the normal life cycles of a cell. But they need nutrients to survive, just like a healthy cell. So tumors attract blood vessels to feed itself, just like any other organ in the body.

When cancer spreads throughout the body, it’s called metastasis.

What causes cancer?

Smoking contributes to up to 30 percent of cancer deaths. Tobacco smoke is the most lethal carcinogen in North America. Diet accounts for another 30 percent of cancer deaths. Saturated fats, high salt intake, and alcohol consumption are the primary culprits. Pathogens account for about 15 percent of cancer deaths, including viruses such as:

  • Human papillomavirus (HPV) is known to cause cervical cancer and some oral cancers. There are 20 million Americans infected with HPV and 75 percent of those people are between age 15 and 49. Cervical cancer is the fifth most deadly cancer in women worldwide.
  • Human herpesvirus 8 (HHV8) is associated with Kaposi's sarcoma and other diseases.
  • Hepatitis B and C are associated with liver cancer.
  • Epstein-Barr virus is associated with Burkitt’s lymphoma.

Cancer can also be caused by bacteria such as Helicobacter pylori, which is associated with stomach cancer. Other causes of cancer include radiation, hereditary factors, steroid hormones, population demographics, and environmental and occupational chemicals. See the chart below for a list of chemicals that are human carcinogens and for which exposure if mainly occupational.


How do cancers get their names?

Cancers are named according to the type of tissue it originates in, and what type of cell it starts in. For example, ductal breast carcinoma.

  • Carcinoma is cancer of the skin and inner membranes in the body (lungs, stomach, intestines). It accounts for 90 percent of human cancers.
  • Sarcoma is cancer of tissue that supports and connects parts of the body (muscle, bone, cartilage, and fat). It accounts for less than two percent of human cancers.
  • Leukemia and lymphomas are cancer of the blood-forming tissues, accounting for about 8 percent of human cancers.

What are early warning signs of cancer?

Be on the lookout for these symptoms that may be warning signs for cancer. While any of these symptoms can have several different causes, it would be good to have it checked out by your doctor.

  • Changes in bowel or bladder habits
  • A sore that does not heal
  • Unusual bleeding or discharge
  • Thickening or a lump in the breast or any other part of the body
  • Indigestion or difficulty swallowing
  • Change in appearance of a wart or mole
  • A nagging cough or hoarseness


Reviewed 1/8/13 by Karen Peterson, PhD, Director of the Office of Scientific Career Development and the Scientific Ombudsman, Fred Hutchinson Cancer Research Center